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Sensors and Materials, Vol. 36, No. 9 (2024) 3843–3849
MYU Tokyo
S & M 3770

System Solution for Waterborne Wood Paint Discoloration
Wen-Cheng (Agingu) ShihNeogeneration Biotech Co., Ltd.,
3F, No. 431, Dadun S. Rd., Nantun Dist., Taichung 408024, Taiwan(Received July 8, 2024; accepted August 26, 2024)

Keywords: waterborne wood paint, discoloration, tannin staining, titanium dioxide

The transition from solvent to low volatile organic compound and eco-friendly paint is a
megatrend in the Asia Pacific region, and waterborne wood paint is still in the early stage of thistransition. The most challenging technical problem is paint discoloration that can serve as asensor for the malfunction of the paint system caused by tannin staining. This is a brownish or
tan discoloration on the paint surface owing to the migration of tannin from the wood substrate
through the paint film. A typical remedy is to apply an oil sealant in the presence of a tannininhibiting agent or a cationic resin in a base coat. This may lead to the constraints of multiple
paint applications constraints, additional paint makeup steps, and unwanted chemical substancemanagement. The described waterborne wood paint that is formulated with alumina surfacecoated TiO2 (Al–TiO2) in combination with an anionic acrylate resin system demonstrates
exceptional tannin stain resistance in comparison with silica/alumina surface-coated TiO2 (Si–Al–TiO2). A good compatibility with an acrylate resin system and a strong affinity towardtannin for alumina on a pigment surface are instrumental in mitigating waterborne wood paintdiscoloration.

1.

Introduction

The Asia Pacific region has seen the fastest growth in the global wood paint market. Thisgrowth is mainly attributed to intensive investments on building and construction industries toaccommodate the growing population, per-capita income, and improved life quality.
The global wood paint market continues to be dominated by the solvent-borne system at 40%
and the high-solid solvent-borne paint system at 23% (solvent-borne system up to 60% in total),
whereas waterborne systems and radiation-curable paints are both at 18% (close to 40% in total).
However, the waterborne conversion ratio in the China market is considered far below 10% incomparison with the global ratio of 18%. Therefore, there is a strong momentum toward ecofriendly products with the Chinese government’s strong push to reduce volatile organic
compound emission and PM 2.5 air pollution with the attempt to promote up to 60% green
paints, such as a waterborne paint system.
*Corresponding author: e-mail: agingu.shih@gmail.com

https://doi.org/10.18494/SAM5230

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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In comparison with a solvent-borne system, the technological challenges of a waterborne
system include limitations on paint application conditions under considerably strict atmospherichumidity/temperature, paint film deformation by grain raising caused by moisture
contamination, the expansion/contraction of the wood substrate, and an additional sealing layer
to inhibit the migration of tannin/other chromophore extractives that cause paint discoloration.
Wood is a complex natural organic material that is mainly composed of two groups ofcompounds: carbohydrates such as cellulose and hemicelluloses (65–75%) and phenols such as
lignin (20–30%). Wood also consists of small amounts of extractives in the form of inorganic
minerals at 0.1–1% and organic extractives at 1–30%, which are chromophores such as tannins,
flavonoids, essential oils, fats, resin, wax, gum, and starch.
Tannins, e.g., tannic acid, are water-soluble, polyphenolic, and high-molecular-weightmolecules that are found in many types of wood, including western red cedar, redwood, and oak.
The brownish color is caused by their conjugated chemical structure, and their water solubility
arises from their polyphenolic structure. The above-mentioned facts about tannins lead to thepotential causes of the waterborne wood paint discoloration.(1)The pH of the waterborne wood paint system is commonly controlled to be slightly alkaline;
therefore, tannins with a pKa of 4.2 are prone to deprotonate and form anionic species. This
characteristic sheds some light on blocking tannin stain migration through the paint film via
electrostatic adsorption by a cationic resin system and/or the addition of chelating additives to
produce a water-insoluble tannin-containing complex.
Paint discoloration by tannin staining may occur when tannins dissolve in water, diffuse tothe interface of the wood and paint film, and then further migrate to the painted surface,resulting in an unsightly yellow or brown discoloration.
The wood bleeding through white paint is a brownish or tan discoloration on the paintsurface, especially on knot holes. This color change on a white paint surface serves as a sensor
for the malfunction of the paint system that cannot effectively mitigate the tannin migrationfrom the wood substrate through the paint film, i.e., tannin bleed-through.
There are two effective approaches to inhibiting tannin stain formation: the barrier approachand the chemical approach.
The waterborne wood paint discoloration, i.e., tannin stain, can be effectively mitigated bypreventing tannin migration from the wood substrate to the paint film surface through (1) thebarrier approach by improving the paint film integrity to eliminate crack formation in the dry
film and (2) the chemical approach by the interaction/chelation with a tannin to produce a waterinsoluble complex, which would not bleed through a waterborne paint film.
As for the barrier approach, two component solvent-borne primers or sealers are typicallyused. These primers/sealers require hazardous isocyanates for curing. Thus, they are notpreferred because of safety, health, and environmental concerns related to the solvent andisocyanate cross-linker. Furthermore, the paint film integrity caused by the well-dispersed TiO2
in a resin system, as shown in Fig. 1(a),(2–6) can effectively eliminate the crack formation in thedry film by producing a resin-pigment composite that contributes to tighter film formation andprevent tannin migration from the wood substrate to the paint film surface as shown by the redarrows in Fig. 1(b).

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Fig. 1.

3845(Color online) Barrier approach via better pigment spacing and its effect on tannin stain resistance.

With regard to the chemical approach, two alternatives are adopted in relation to theinteraction with tannin as described below.
(1) Inorganic chelating metals, which are slightly water-soluble metal cations, such as Ba2+, Zn2+,
Al3+, and Zr4+, react with hydroxyl groups on the phenolic rings of tannins.(7)(2) Cationic resins can cause water-insoluble complex formation, since most of the water-soluble
extractives in wood are anionic in nature.(8,9)
However, these two approaches suffer from some limitations that most paint producers avoid,which include heavy odor, poor shelf life, comparatively high paint makeup cost with theaddition of a metal-chelating agent, and spray gun blockage with the use of a cationic resin
during paint application. Therefore, an existing anionic acrylate resin system in the presence ofminimum tannin-inhibiting additives and the optimized TiO2 pigment is a potential alternative
to achieve the maximum benefits in terms of both paint performance and paint makeup cost.

2.

Data, Materials, and Methods

Exceptional paint performance in terms of whiteness, hiding power, and gloss withadditionally improved tannin stain resistance was achieved through a unique synergistic effectbetween Al–TiO2 and a compatible resin system. The paint formulated with alumina surfacecoated TiO2 (Al–TiO2) in combination with an anionic acrylate resin system (NeoCryl® XK-85
from DSM is a small-particle-size styrene anionic acrylic emulsion designed for coatingsrequiring outstanding corrosion, water, and humidity resistances) demonstrates prominent
tannin stain resistance in comparison with silica/alumina surface-coated TiO2 (Si–Al–TiO2) asshown in Fig. 2. The specified ingredients for the testing formulation include water, a dispersant,a white TiO2 pigment, resin, a coalescing agent, and a thickener in the absence of a tannininhibiting agent (HALOX BW-100 from ICL).

3.

Results and Discussion

The paint with controlled film thickness and viscosity was applied by a spray technique on anoak substrate. After aging indoors for 1145 h, the delta E (∆E) of the paint was evaluated.
Furthermore, the paint system made from Al–TiO2 in combination with an anionic acrylate resin

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system can approximately meet the required tannin stain resistance that was formulated with acationic resin system (NeoCryl® XK-350 from DSM is an acrylic cationic copolymer emulsion
designed for primers against knot bleeding), as shown in Fig. 3. As mentioned earlier, a cationicresin is commonly used to prevent tannin migration. Here, ∆E is calculated to indicate thechange in paint color after aging indoors for 1145 h as measured in the Hunter Lab color space

Fig. 2. (Color online) Discoloration of paints made from (a) Si–Al–TiO2 and (b) Al–TiO2 in anionic acrylate
systems on an oak substrate.

Fig. 3. (Color online) Tannin stain resistance defined by ∆E for Si–Al–TiO2 and Al–TiO2 in cationic and anionicresin systems.

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on a 3D axis at L*, a*, and b*. A low ∆E indicates a small difference between the measured color
after 1145 h and the original paint color, and it stands for a higher tannin stain resistance. The
method strictly followed ASTM D6686 for the “Standard Test Method for Evaluation of Tannin
Stain Resistance of Coatings.”
Two determining factors for achieving the prominent tannin stain resistance for the paint thatwas formulated using Al–TiO2 in an anion acrylate resin system include a good compatibility
between Al–TiO2 and an acrylate resin system and an effective interaction with tannin throughalumina on a pigment surface.
Regarding the interactions between an acrylate resin and Si–Al–TiO2/Al–TiO2 pigments,
Zaman et al.(10) investigated the bonding type and absorption strength of polyacrylic acid (PAA)on silica, alumina, and kaolin. They found that the PAA adsorption on the surface of alumina
was found to be approximately 10 times stronger than that on kaolin particles, and there was noadsorption of PAA on the surface of silica particles, i.e., PAA adsorption strength: Al > kaolin>> Si. The carboxylate oxygens (R-COO−) can bridge an Al3+-octahedral dimer, which consistsof Lewis acidic heteroatoms to accept the electrons from carboxylate groups.(11,12) The strong
adsorption of the acrylate resin with carboxylate functional groups on an Al surface-coated TiO2pigment(13,14) can improve the paint film integrity(15–19) that effectively eliminates tannin
migration.
To investigate the interaction of tannin with the specified surface-coated pigments, a tannin
retention test was conducted on both Si–Al–TiO2/Al–TiO2 pigments. The aqueous tannin acidsolution was mixed with a specified pigment slurry. The mixture was sonicated for 5 min andthen centrifuged. The upper clear solution was removed and fresh water was used to flush thepigment. These steps were repeated three times, and after the third flushing, the solution wascollected and subjected to transmittance measurement. The tannin% in solution was calculated
at a wavelength of 400 nm by UV-visible spectroscopy. The low concentration of tannin acid in
the flushed solution suggested that more tannin acid was adsorbed by the pigment. The Al–TiO2pigment imparted a stronger capability than the Si–Al TiO2 pigment for tannin interaction, asshown in Fig. 4.

Fig. 4.

(Color online) Tannin acid retention on Al–TiO2 and Si–Al–TiO2.

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The design concept for the entire paint system was applied to the commercial wood paintformulation in collaboration with a wood paint producer and a dispersant supplier. Currently, thework is still under way.

4.

Conclusion

Tannin stain can cause paint discoloration, which serves as a sensor for the malfunction ofthe paint system. The described waterborne wood paint that is formulated with alumina surfacecoated TiO2 in combination with an anionic acrylate resin system demonstrates exceptionaltannin stain resistance in comparison with that formulated with silica/alumina surface-coated
TiO2. A good compatibility with an acrylate resin system and a strong affinity toward tannin foralumina on a pigment surface are instrumental in mitigating waterborne wood paintdiscoloration.

Acknowledgments
I am grateful to Nana Chiang, Sarah Ko, and Herman Lim for their excellent experimental
work.

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About the Authors
Wen-Cheng (Agingu) Shih received his Ph.D. degree from Taiwan
University, Taiwan, in 2007. After that, he has played multiple roles in R&D,
technical marketing, and operation in the titanium dioxide industry. His
research interests are in heterogeneous catalysis and surface modifications of
inorganic oxide on pigment. (agingu.shih@gmail.com)